Method and tooling for z-axis offset of lead frames

ABSTRACT

A method and apparatus for enabling z-axis offset of narrow metal ties straps in lead frames used for packaging integrated circuits to prevent bowing or distortion. Simultaneous offsetting of the tie strap and stress relief mechanisms are provided on both the front and back sides of the lead frame. Those mechanisms include indentations along the long or primary axis of each tie strap, coupled with depressions across the top surface both at the center of the lead frame and between the base of the off set and the chip attach locations to prevent bowing in small pad and no pad lead frames, in particular.

This application claims the benefit of Provisional application No.60/157,780 filed Oct. 5, 1999.

FIELD OF THE INVENTION

This invention relates generally to the field of metal forming, and moreparticularly to the forming of lead frames used in the assembly of microelectronic devices.

BRIEF DESCRIPTION OF PRIOR ART

Integrated circuit devices, having an integrated circuit chip and a leadframe which are sealed within a protective enclosure find wide use inproducts, among which are consumer electronics, computers, automobiles,telecommunications and military applications. A means to electricallyinterconnect an integrated circuit chip to circuitry external to thedevice frequently takes the form of a lead frame. The lead frame isformed from a highly electrically and thermally conductive material,such as copper or copper alloys. The lead frame is stamped or etchedinto a plurality of leads, and a central area, called a chip pad, onwhich the integrated circuit chip is attached. The chip is electricallyconnected to the leads, usually by wire bonding, and the device isencapsulated to provide mechanical and environmental protection.

Lead frames typically include a solid chip pad somewhat larger than thechip to which the integrated circuit chip is attached by an adhesive oralloy. However, currently many lead frames 101 are fabricated with asingle or multiple small circular pads 102 as shown in FIG. 1, or simplyto strips of metal where the chip is attached, and the large chip pad iseliminated. The chip pads 102 are connected to outer support rails 105by thin etched or stamped extensions of the metal, called tie straps106. Support rails 105 also hold together one or more lead frames in astrip until encapsulation is completed.

Those lead frames having one or more small pads 102 as in FIG. 1 aretypically referred to as S-pad or small pad lead frames. A small,circular pad is positioned approximately mid-way from the edge of thetie strap to the center of the lead frame where the tie straps intersect108. The chip is positioned atop the frame and the unpatterned side ofthe chip attached to the pads by an adhesive. An outline of a chipposition is represented by the dashed line 103.

As shown in FIG. 2a, lead frames 201 may be attached to the activepatterned surface of the chip 203, as in devices referred to as LOC orlead-on-chip, and illustrated in FIGS. 2a and 2 b. A chip is attached toa flat portion of the lead frames itself, and most often to a down-setor offset portion 209 of the frame.

Lead frames having a reduced chip pad area were developed in response toa failure mechanism in surface mount packages often referred to as “popcorning”. Moisture ingress into the plastic package is trapped betweenthe chip and the metal chip pad, and when subjected to a rapid thermalexcursion, such as solder attachment to a printed wiring board, thevapor pressure causes the plastic package to bulge and sometimes crack.This failure mechanism can be avoided by eliminating the large solidmetal die pad.

In the process of lead frame down-setting a selected strip of metal iselongating in a die under pressure from either a hydraulic orelectrically driven press while the support metal remains planar. Themetal in two or more tie straps metal is forced downward to form angledbends and is pressed toward the center of the lead frame by using aforming punch to press the tie straps against the die surface. The moreductile metal, following the path of least resistance moves toward thecenter where the tie straps converge, and lacking a relief mechanism,the metal strip bows. FIG. 3a is a schematic of a lead frame 301 withbowed tie strap 308 at the center where the tie straps converge. Thesemiconductor chip 302 is attached to the small pads 303 by adhesive 304only at localized area owing to the non-planar attach area. Theschematic in FIG. 3a demonstrates a device bowed in a concave direction.Convex bowing is equally as big an issue for the assembly ofsemiconductor devices.

In conventional lead frames having a rigid or solid chip attach pad inthe center, stress is relieved in the large pad, and bowing is much lessof a problem. However, in the case of small pad frames where only smallrelief areas are provided, the narrow metal frequently is distorted atthe location where pressure converges.

However, small or no pad lead frames are not without significantmanufacturing challenges. One of the more difficult issues has beenwarping or distortion of the long thin tie straps which occurs duringoffsetting the chip mount area. Either convex or concave bowing of thetie strap and chip attach areas prevent the chip from seating correctly.

Owing to the reduced contact area for chip attachment, it is imperativethat the chip attach area be planar and allow the small amount ofadhesive on the pads to contact and firmly seat the chip. Lossesresulting from bowed or distorted lead frames result in not only yieldloss, but also present a reliability concern for both mechanicalstresses on wire bonds, as well as diminished thermal transport path. Asolution to this issue has been sought by the industry since theinception of small pad and no pad packaged devices.

SUMMARY OF THE INVENTION

As thinner integrated circuit packages, and consequently thinner leadframes are demanded by the industry, warping and distortion of the leadframe chip attach area has becomes a more prevalent issue. Further, theheavily favored lead frame materials are alloys of copper because of itsexcellent thermal conductivity and ease of processing, but themalleability and ductility of these copper alloys allows greater warpingthan other more rigid metallic alloys.

It is an object of this invention to provide an essentially no cost,permanent, and consistent means of eliminating bowing and distortion oflead frame tie straps resulting from the z-axis down-setting process.

It is an object of the invention to provide tooling which is common to alarge family of devices, and the methodology for altering the toolingdesign to be applicable to many other lead frames sizes and shapes.

The chip attach area of lead frames is offset from the plane of the leadframe in order to minimize the length and looping height of bondingwires by positioning the chip surface at more nearly the same level asthe lead frame bond fingers to which bond wires are attached. Offset orz-axis down-set is accomplished by positioning the stamped or etchedlead frame in a press having a down-set die and a forming tool. Thematerial to be offset is elongated by the contact and pressure from theforming tool, thereby stretching the metal and increasing its lengthwith respect to the surrounding planar portion of the lead frame. Themetal is being elongated or stretched along the surface of the formingdie from opposite sides of the lead frame and converges toward thecenter of the lead frame. This results in the central area bowing ineither a concave or convex direction, either of which is unacceptable.

The present invention provides a set of inserts to be positioned in thedown-set die which have a protrusion above the die surface in the centerof each tie strap. In addition, a series of protrusions are fabricatedon the down-set forming punch. The insert tooling forms an indentationon the backside of the tie strap which allows the lead frame material tobe pushed upward, and to move to the outer edge of the tie strap. In thesame operation, protrusions on the forming punch create small lateralimpressions on the top surface of the tie strap which control the flowof material being pushed toward the center of the lead frame duringdown-setting. Simultaneously creating small controlled indentations inthe top and bottom surfaces, as well as along both the longitudinal andhorizontal axis allows relief for the deformed lead frame material, andresults in a flat, planar chip attach area.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plane view of a prior art “S” or small pad lead frame.

FIG. 2a is a cross section of a known LOC package.

FIG. 2b is a plane view of a known LOC type lead frame.

FIG. 3a is a cross sectional view of a prior art down-set “S” pad leadframe.

FIG. 3b is a cross sectional view of a down-set “S” pad lead framefabricated using the method of the current invention.

FIG. 4a is a planar schematic view of a die insert of the currentinvention.

FIG. 4a′ is a cross sectional view of a die insert of the currentinvention.

FIG. 4b is a schematic view of the die surface with insert locations.

FIG. 5 provides a schematic of a forming punch tool with protrusions andlocations.

FIG. 6 is a plane view of the top surface of an “S” pad lead framehaving impressions from forming punch protrusions.

FIG. 7 is a plane view of the bottom surface of an “S” pad lead framehaving indentations from the die inserts.

FIG. 8 shows the relative positions of stress relief mechanisms on an“S” pad lead frame

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, there is provided a method toeliminate bowing or distortion of lead frames resulting from offsettingthe tie straps, including the first step of providing the necessaryforming tools Detailed descriptions of the preferred embodiment areprovided herein. It is to be understood, however, that the presentinvention may be embodied in various forms. Therefore, specific detailsdisclosed herein are not to be interpreted as limiting, but rather as abasis for teaching one skilled in the art to employ the presentinvention in virtually any appropriate detailed system, structure ormanner.

A preferred embodiment of the present invention uses conventionalhydraulic or electrically driven presses for offsetting lead frames, incombination with a novel offset tooling set. The improved tooling set ofthe current invention simultaneously relieves stress on the bottomsurface and the top surface of the lead frame tie strap during thedown-setting process and avoids build-up and bowing of the metal towardthe center of the work-piece. FIG. 3b illustrates the chip 312 attachedto small chip pads 313 in a lead frame 311 processed using the formingtooling of the current invention, wherein the tie strap, the small pads313 and the central area 318 where tie straps converge is flat afteroffset forming.

FIG. 4a and 4 a′ are planar and cross sectional schematicrepresentations of one of a set of tools to be insert into the surfaceof the down-set die. Each insert tool 401 comprises a steel devicehaving a rib 402 which protrudes about 0.0005 inches above the surfaceof the die, and corners 404 shaped to conform to those of a cavity inthe die. FIG. 4b shows the locations of cavities 411 in the die surface410. One rib shaped tool 401 is provided to be positioned in each cavity411 between the angled down-set areas 412, and the small pad of the leadframe (not shown). The preferred embodiment provides a cavity 411 in thedie for each pad of a four (4) small circular pad device.

Length and width of the protrusions 402 on the insert tools 401 aredetermined by the specific lead frame dimensions. The length is derivedfrom the location of chip pads and the base of the downs set, and willextend inside those boundaries. In the preferred embodiment the tool isin about 0.12 inches long. Width of the protruding rib is dependent uponthe width of lead frame tie strap, and the width 403 of the protrusionis about 0.0002 inches wide for a tie strap of nominal width 0.012inches.

Protrusions 402 on the inserts force the ductile lead frame metal tocreate an indentation on the bottom side along the primary axis of thetie strap, and to flow toward the outer edge of the lead frame metal,rather than simply moving toward the center of the device.

The surface of the forming punch, as shown in FIG. 5 has a series ofdown set angles 504 which press the lead frame tie straps onto the diesurface, thereby forming a down-set area out of plane with the supportstraps. The downs-set angle is 30 degrees and depth of the down-set inthe preferred embodiment is about 0.009 inches. The forming punch of thecurrent invention includes five (5) protrusions 502/503 of about 0.0005inches height on the surface of the tool. There are 4 small protrusions502, each of which contacts one of the tie straps, and a centrallylocated large protrusion 503 which contacts all of the tie straps. Theseprotrusions correspond to the locations where impressions are formed onthe top surface of the tie strap. An array of four (4) protrusions 502,about 0.015 to 0.016 inches on a side create a horizontal depression,about 0.015 inches long in the tie strap near the edge of each smallcircular chip pad, on the side toward the angled down-set. Theseimpressions extend the full width of the tie strap, and areperpendicular to the backside indentations. The fifth protrusion 503 onthe forming tool is a square positioned at the center of the tool sothat one side intersects each of the tie straps, and makes a depressionfrom a location near the edge of the circular chip pad across the centerof the lead frame to near the pad on the opposite tie strap. Protrusionson the forming punch are about 0.0003 inches in height, and spacing isrelated to the location and size of the circular die pads. In thepreferred embodiment, the square protrusion is about 0.13 inches, andwill form an impression extending radially about 0.065 inches inwardfrom the center of the tie strap intersection.

The impressions on the top surface of the lead frame horizontal to thelength of the tie strap, coupled with the longitudinal indentation onthe backside of the tie strap allow the metal to be moved laterallytoward the edges on the back side, and provide a horizontal relief stopfor the metal as it moves toward the center on the top side.

FIG. 6 illustrates the top surface 610 of a small pad lead frame havingdepressions 602/603 as created by the forming tool protrusions 502/503in FIG. 5. Each impression extends the width of the tie strap, and isabout 0.0002 to 0.0003 inches deep. Shaded areas 602/603 represent thedepressed area caused by protrusions 502/503 on the forming tool. Theoutline of protrusions 502/503 is represented by dashed lines 502 a/503a in FIG. 6.

FIG. 7 is a plane view of the back surface of the lead frame 710.Indentations 711 along the primary axis of the tie strap 712 result frompressing the tie straps against ribs 402 on the form die insert as shownin FIG. 4a. Indentations 711 are in the range of 0.00035 to 0.0005inches in depth, and are located between the termination of the down-setangle termination 715 and the circular chip pad 713. Indentations aredesigned to terminate prior to the onset of impressions 602 on the topsurface in FIG. 6.

FIG. 8 illustrates the relative locations of the top surface depressions802/803 and the bottom surface indentations 811 with respect to thetermination of the down-set angle 815 and the circular chip pad 813 inone half of a lead frame 820 cross section. The center of the lead framewhere tie straps converge is noted by the dashed lines 821.

It should be understood that within a standardized lead pitch and for aspecified number of leads on a lead frame, the small pad lead frameaccommodates a large range of chip sizes. For example, a 132 pin quadflat pack lead frame may be used for many chip sizes and types, andtherefore, the embodiment of this invention is applicable to a largenumber of Integrated circuit devices.

It should further be understood that precise dimensions of theindentations and impressions are dependent upon the lead framedimensions, and that tooling dimensions are different for different leadframes, but the relative locations are similar, and therefore theinvention is applicable to the entire family of small pad lead frames.

It should further be understood that the preferred embodiment describeda tooling set and application including four (4) small circular chippads, but that the invention is applicable to devices having a singlecircular or other shaped chip pads by omitting the center protrusion onthe forming press. It is further applicable to LOC (lead-on-chip) or COL(chip-on-lead) lead frames where there are no chip pads, or toconventional pad devices where distortion of the ties strap is aproblem. Such devices include those having a deep down-set angle, longtie straps or very thin or ductile material.

It should also be understood that because the stress relief mechanismsof forming depressions in both the front and back side of the lead frameare created during the forming process, the solution is permanent, asopposed to a solution where the stress corrected, but may return as afunction of memory in the metal.

Because the stress relief mechanisms are created by hard tooling withlimited tolerance ranges, the relief mechanisms are reproducible andlimited by tooling tolerances.

What is claimed is:
 1. A method for offsetting a narrow ductile metalpart wherein the offset section is stress relieved, the methodcomprising the steps of: providing a form die with protruding ribinserts along the primary axis of said metal part, providing a formingpunch with protrusions horizontal to said die protrusion ribs,positioning said metal part on the surface of said form die, andaligning said forming punch to the die, then applying pressure to saidpunch to form an off-set plane with stress relief depressions in themetal by the die protrusion ribs and the protrusions.
 2. A method as inclaim 1 wherein said metal part is a lead frame for interconnecting asemiconductor device.
 3. A method as in claim 1 wherein said inserts andsaid forming punch comprise steel.
 4. A method as in claim 1 wherein theapplied pressure is in the range of 300 to 500 pounds force.
 5. A methodas in claim 1 wherein said metal parts comprise copper.
 6. A method asin claim 1 wherein said form die in positioned in a hydraulic press. 7.A method as in claim 1 wherein said form die is positioned in anelectrically driven press.
 8. A method as in claim 1 wherein stress onsaid metal part is relieved by longitudinal indentations on the backside and concurrently by horizontal depressions on the front sidearrayed between the bottom of the down-set and the center of the metalpart.
 9. A method as in claim 1 which is applicable to metal partshaving different shapes and sizes.
 10. A method as in claim 1 whereinstress relief mechanisms of indentations are permanent.
 11. A method asin claim 1 wherein said stress relief mechanisms are accurate andreproducible.
 12. A method for offsetting a tie strap in a lead framepart wherein the offset section is stress relieved, the methodcomprising the steps of: providing a form die comprising steel withprotruding rib inserts along the primary axis of said tie strap,providing a forming punch comprising steel with protrusions horizontalto said die protrusion ribs, positioning said lead frame part on thesurface of said form die, then aligning said forming punch to the diepositioned in a press, applying 300 to 500 pounds force, andsimultaneously creating an off-set plane while creating permanent stressrelief depressions in the tie strap comprising longitudinal indentationson the back side and horizontal depressions on the front side arrayedbetween the bottom of the offset and the center of the metal part.
 13. Amethod for offsetting a tie strap in a lead frame part wherein theoffset section is stress relieved, the method comprising the steps of:providing a form die with a protruding rib having a lenoth with aprimary axis extending along the primary axis of said tie strap;providing a forming punch; positioning said lead frame part on thesurface of said form die; aligning said forming punch to said die; andapplying force to said punch to press the protruding rib against the tiestrap.
 14. The method of claim 13, wherein said step of providing aforming punch comprises providing a forming punch with protrusionshorizontal to said die protrusion ribs.
 15. The method of claim 13,wherein said step of providing a form die comprises providing a steelform die.
 16. The method of claim 13, wherein said step of providing aforming punch comprises providing a steel forming punch.
 17. The methodof claim 13 wherein said step of applying force to said punch comprisesapplying approximately 300 to 500 pounds force to said punch.
 18. Themethod of claim 13, wherein said lead frame part is a small pad leadframe.
 19. The method of claim 18, wherein said small pad lead framecomprises a small circular pad.
 20. The method of claim 13, wherein saidlead frame part is a lead-on-chip lead frame.